In optical communication systems, information is carried along a distance using light. In some applications, the light travels over a free space. In these free space applications, a highly collimated light source, such as a laser, is used. In other applications, the light is carried by an optical fiber. The optical fiber comprises a flexible, transparent fiber made of silica.
In some applications, the optical fiber may comprise a single-mode fiber, where the optical fiber carries light only directly down the fiber, i.e. the transverse mode. In other applications, the optical signal may comprise multiple modes, i.e. being carried by a multi-mode optical fiber. Although traditional multi-mode fibers do not provide greater bandwidth than single-mode fibers, as they are limited by modal dispersion, they are used because of mode path redundancy, and therefore tolerance to connector and interface imperfections.
In some optical applications, it may be desirable to manipulate individual modes of the multi-mode optical signal, i.e. a few mode optical signal. In these few mode applications, there is an approach to the dispersion limit issue of multi-mode applications, as each mode is multiplexed as a separate data stream. The primary advantage provided by few mode signal transmission is overall signal bandwidth of the fiber, as each mode acts as an independent signal carrying path in parallel with the other mode paths. For example, in the multi-mode optical fiber, an application may need the multiple modes of the optical signal carried thereon to be separated out for processing. In these applications, the optical signal is typically split into multiple paths, each path comprising a mode filter for passing a respective mode. This application may be helpful in mitigating bandwidth bottleneck issues, by breaking the high bandwidth signal apart.
One example of an optical filter is disclosed in U.S. Pat. No. 7,110,646 to Eggleton et al. This optical filter comprises a length of fiber having a core with a certain refractive index, a cladding peripherally surrounding the core with a refractive index less than the refractive index of the core, and at least one hollow region disposed within the cladding in proximity to the core or within the core itself. The optical filter includes a liquid controllably moved within the hollow region for modifying the effective index of the fiber to tune its characteristics.